Medical Autoclave Stress Analysis and Fatigue Life Prediction

Mechanika has recently completed a static stress analysis and detailed fatigue analysis for an autoclave, which had been designed and prototyped by a client in the medical instrumentation sector. The project consisted of full substantiation and compliance with “BS EN 13445-3: Unfired Pressure Vessels – Design”, supplemented by detailed fatigue data from Mechanika Ltd.’s expansive fatigue curve library.

The output to the project was a 70 page stress analysis report, which documented all of the load cases, stress hand calculations, FEA, fatigue analysis, including a reserve factor summary table, detailed conclusions and recommendation to enable the client to understand and control every variable in their design.

The autoclave is used for sterilising medical equipment and instruments, using pressurised steam and vacuum pressures in a controlled chamber. The autoclave consisted of a main vessel, steam generator and mechanised door, all of which were assessed by Mechanika.

Load cycling was plotted, and the associated stress ranges, mean and alternating stresses were calculated using FEA and hand calculation methods. The extracted results were compared against the fatigue curves and the cumulative damage for all cycles was calculated.

A combination of shell and solid quadrilateral elements was used to model the geometries analysed with FEA. Further sub-modelling was undertaken for stress linearization and extraction of stress components for the fatigue assessment.

Internal pressures and nozzle loads were applied to the vessel and generator. The FEA model used a unit loading method, whose results were scaled depending on the cycle pressure being considered.

The post processing of the models required accurate extraction of the principle and shear stress components as well as bending and fatigue stress components depending on the clause being satisfied from the design code.

The Door mechanism was assessed using hand calculations and detailed FEA, to determine the fatigue life of the components and the maximum deflection, to ensure that the door seal was not compromised.